1. Field of the Invention
The present invention relates to an image processing apparatuses, and more particularly, to an image processing apparatus that carries out a process of enhancing the contour of an image on image data sent from an image reader that reads out an original image.
2. Description of the Related Art
In a conventional image processing apparatus, differential image data is extracted from the original image data using a Laplacian filter as disclosed in Japanese Patent Laying-Open No. 6-152960. The data is converted according to a lookup table (LUT) into contour enhanced image data. A contour enhancement process is carried out by adding the contour enhanced image data to the original image data. Application of a contour enhancement process can be suppressed according to the result of a region determination for each pixel data forming the image data.
Japanese Patent Laying-Open No. 6-152929 discloses the technique of expanding the readout image by an arbitrary amount in the main scanning direction and the sub scanning direction of the image reader to reduce the problem of thinning, blurring, and disconnection in the text and image.
However, increasing the detection level of the contour to be enhanced will cause the difference in density between the contour portion such as a character and the region other than a character to be increased. The reproduced image was extremely poor in picture quality since the characters subjected to a contour enhancement process looks as if the character was significantly bordered. If the detection level of the contour to be enhanced is reduced for the purpose of preventing disconnection in the lines, the portion other than the thin lines of characters having a gentle image density gradient will also be determined as the contour portion of a character. Image noise will therefore be increased.
If a LOG correction is carried out on the image data read out by the CCD, the image noise in the region of high density is enhanced. Particularly in the case where the image data of the portion corresponding to the ridge of density for a line with a small width exhibits asperity (distribution is not uniform), this irregularity will be further deteriorated by the contour enhancement process. In the reproduced image, the thin line will become disconnected, in addition to the above-described problem.
There is also a problem that, when noise is included in the image, the noise will be eventually increased by the contour enhancement process.
In the latter, if the image is simply expanded, the configuration of the edge of the characters will be altered. When a contour enhancement process is carried out after the expansion process, there is a possibility that the resolution is degraded.